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I'm going to try and attach some images. I'll try to describe the setup. On top of the air handler is a "Y" where two 16" flex ducts are connected. The first split runs a short distance to a ductboard plenum(not sure if that is the right word or not, but it is duct board where all the flex duct feeds to each vent connect). This part of the split supplies air to 50 or 60 percent of my house. The 16" ducts in the picture are the ones that run across my unfinished bonus room and to the far side of the house. This supplies air to the remaining 40 or 50% of the house, and splits into a similar plenum as the one closer to the air handler.

I have a 4" aprilaire filter which I have updated to the most recent slide in filter type. The unit tends to perform well, although on super hot or super cold days, it does struggle to maintain or change the temperature. My electric bills have always been reasonable (I average about $250/month in North Florida in an all electric house with rates about .12/kwh, with a 2850 sqf house). The only room where we ever have a problem is my daughters room on the far side of the house with 3 exposed walls.

I like the turning vane suggestion. Without bottom side or side side and 2 filters you are airflow challenged above 3 ton, and doesn't seem room for return modifications. (maybe uDarrel can confirm this) Unfortunately it feels like 3 ton might be too aggressive given your summary of current operation, unless the house has high leakage rate and/or good insulation opportunity that can bring load down.

4 ton might be the compromise you live with (and is probably what your restricted 5 ton was providing). A mod (vna) means it won't choke but for very hot days and if you ask for recovery. It'll probably exist around 2 ton most of the time.

By the looks of things you can afford good long term decisions, and sounds like thats how you already think. So can you research energy audits in your area? An understanding of leakage and load calc would be really nice, particularly with such tight sounding parameters.

Not sure if anyone else sees poor transition to aprilaire. Not that you really could do any better. And with a 5 ton you would will still struggle with one aprilaire. Even though IMO it is the best performer in the airflow Dept.

Also please describe the ducts that hook to top of return riser and the indoor return grille set and do you run filters inside?

Also a very quick and easy static pressure test between air handler and that aprilaire would tell us a lot.

The ducts on the return are similar to the supply, with a Y at the top feeding to two 16" flex lines. Those flex return lines hook up to 2 separate plenums (one on each side of the house). There is one large return (2'x2' maybe) in the main hall, another return of the same size in the living room, and then small returns (1'x1' perhaps) in each of the 4 bedrooms. I'm guessing the static pressure test between the air handler and the aprilaire is not something I could perform since I wouldn't even know where to begin as a homeowner. There are no other filters in the system outside of the aprilaire.

I've got a 5 ton Carrier Heat Pump (38YDB outside unit, FV4B indoor unit), that has developed an evaporator coil leak. I believe my system is fairly efficient already (unable to find my combo in the ahri directory), so I'm wondering what my best plan of action is here. The quotes I've received to replace the evaporator coil have all been in the same ballpark, so I don't think I'm getting taken advantage of, but it seems expensive just to replace the evaporator coil. I was given a quote on the Infinity 21 with Greenspeed, and it seems like a really nice unit. I'm looking for advice on whether I should replace the 8 year old coil (an only get a 1 year warranty) and hope for the best, or to go ahead and replace the entire system(and get the 10 year warranty). It seems like my system is at an age where either decision could be correct, and I'm just looking for advice on which route I should take. I live in North Fl if that makes any difference, and the cost to buy the Infinity 21 system is about 4x's the cost of the coil only replacement.

I really appreciate all the responses, there is clearly a wealth of information here. I am going to have another contractor out to take a look at things and give me an assessment, but that begs the question, How do I find contractors who actually know the things that you guys are talking about here? It seems like every time I have someone out, they don't have any of this type information. If I start asking questions from things I've learned here in the past, I just get a blank stare and something along the lines of "I just service the system, I don't know anything about making sure the duct work is sized properly" (or something similar). I tried looking here for a professional from this site, but it looks like the closest one is well over an hour away.